EVOLUTION: THE POGONOPHORA LINK

THE LINK

Pogonophora are the previously unknown link in the evolutionary sequence connecting annelid worms to vertebrates via the hemichordates.  Some items of evidence not mentioned in earlier posts on this topic include information compiled by Libbie Hyman and presented in her 1959 volume on The Invertebrates, volume V, 1959.  The link status demonstrates the error of the widely assumed evolutionary separation of deuterostomes from advanced protostomes.

The confusion of the features of pogonophorans blending features of protostomes and deuterostomes was easily dismissed as caused by lack of adequate availability of well preserved specimens or some such estimation of the recently discovered group.

On page 59, she noted that, prior to the recognition of the pogonophorans as a phylum, one was described as "a vermiform animal with a crown of tentacles and was considered by its describer to be a polychaete annelid of the family Sabellidae".

Also, on page 59 she says "These outstanding studies by Ivanov have shown that the pogonophores are closely related to the hemichordates and belong among the Deuterostomia."

Webb's (1964) discovery of the segmented posterior of a pogonophoran helped endorse their annelid origin.  Later, the contrary view was demonstrated by  Gans and Northcutt (1983) affirming their connection to the deuterostomes despite the obvious annelid connection demonstrated by the generally unknown work of Webb.

So, Gould's (2002) contention that the evidence for elimination of the annelid theory should be dismissed means that the annelid theory should be reinstated as a leading explanation for the inversion of annelid features shown in the chordates.  [  http://evolutioninsights.blogspot.com/2013/06/evolution-protostome-deuterstome-link.html  and  http://evolutioninsights.blogspot.com/2015/05/evolution-annelids-to-chordates-middle.html  ]  The second of the two posts just mentioned illustrates the larval similarity of pogonophoran larvae to solitary hemichordates.  Hyman, on page 219, notes Caullery's 1944 observation that one pogonophoran's developing young "strikingly resemble young stages of the buds of Cephalodiscus", a colonial hemichordate.

I have tried to clarify the protostome-deuterostome connection demonstrated by the pogonophorans in numerous posts on this blog.  By the time I realized the connection existed when provoked by the 1983 assertions of Gans and Northcutt, I had already published two revisions of Hegner's invertebrate zoology text, one in 1968 the other in 1981.

THE EMBRYOLOGICAL SHIFT FROM PROTOSTOME TO DEUTEROSTOME

The stumbling block for accepting the link is the drastic change in embryology that occurs.  I might not have realized how it occurred if I had not revised Hegner's Invertebrate Zoology for its second edition (1968).  In the process I had to deal with the proper classification of pogonophorans and realized the affinity that seemed impossible.

I tend to think there are answers for everything.  I did not have the answer, but the needed facts were already in my background.  As a result of the book preparation I had come to the conclusion that pogonophorans were extremely long-lived. [ http://evolutioninsights.blogspot.com/2015/05/abyssal-ocean-environment-and-extreme.html ]  I did publish that conclusion in a 1978 abstract (Engemann, J. G., Indirect evidence shows deep-sea benthos may reach extreme ages as individuals.  Am. Zoologist, 18:666).

My 1963 doctoral thesis at Michigan State University made me aware of the extreme affects environment can have on selection and evolution.  I had not made much attempt to publish bits of the thesis because they were not greatly different from many other studies.  But it made me aware of some necessary bits of information that helped explain the way embryological shift had been expedited.

The peculiar egg appendage of the isopod Asellus had been illustrated in Sar's work on the crustacea of Norway in the 1800's.  K. H. Barnard had studied phreatoicid isopods in South Africa in the early 1900's and noted a bulge in embryos in a homologous position with the egg appendage that persisted briefly after the embryo hatched.  My sections of phreatoicid eggs showed a thin-walled, yolk-filled homologue of the appendage existed before hatching as described in - http://evolutioninsights.blogspot.com/2014/06/egg-appendages-of-michigan-isopod.html .

The big thing to take from the above is that embryological features can evolve separately from the rate of adult features.  Thus the old presumption that the earliest features in the evolution of the embryology of an organism correspond to the older ancestral stages in its evolution is not always true.

The thesis comparison of Tasmanian and Michigan isopods from comparable latitudes but quite different ecological circumstances in temporary pools showed drastically different rates of development and how r- and K- selection can work before the theory was described (MacArthur, R. H., and E. O. Wilson.  1967.  The Theory of Island Biogeography.  Princeton Univ. Press, Princeton, N.J.  203 pp.).  It made it possible for me to understand the extreme age due to the abyssal conditions caused loss of some features enabling the development of the inverted deuterostomes.  Some of those ideas are in other posts.

THE MOLECULAR DATA

The exquisite work molecular biologists can do is remarkable.  But it is limited and not easily applied to discovering ancestral relationships at the phylum level.  Those limitations have been discussed in the post - http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html  .

My understanding of the slow rate of DNA changes in the pogonophora was confirmed by seeing how they showed up in phylogenetic studies among clusters of disparate groups due to little change in the over half-billion year old group almost in suspended animation in the abyssal sediments.  Unfortunately, the generation time error affecting molecular clocks has not been considered in most work on molecular phylogeny.

MISSING LINKS

What do you call a missing link, such as the pogonophorans, once they are discovered?

CREDITS

I never expected to have work on an obscure invertebrate lead to the above understanding.  But, in retrospect, I realize that much credit goes to unknown biologists and scientists of the past as well as many known ones.  Thanks to Dr. V. Hickman of the University of Tasmania for calling the phreatoicid isopod to my attention, Dr. T. W. Porter of Michigan State University and the rest of my doctoral committee for keeping me focused on the comparative thesis study, the U. S. Fulbright Agency for the award for study at the University of Tasmania, Western Michigan University for allowing me to teach a ridiculous range of classes whose subject matter provided some needed insights, and the Macmillan Publishing Company for contracting with me to revise their texts.  A book would be needed to explain how students, family, friends and strangers contributed to my development under the guidance of the same Creator that made evolution a reality.


Joseph Engemann   Kalamazoo, Michigan      October 24, 2015

EVOLUTION: INSECT SPECIATION

ISOLATING MECHANISMS

Insects have numerous isolating mechanisms that enable sub-populations of species to avoid hybridization and take different evolutionary routes to formation of new species.  Darwin found the isolation of finches on different Galapagos islands may have been responsible for enabling them to evolve specializations leading to different species.  Island populations of insects may produce new species by a similar mechanism.

Geographic isolation and population size

These two factors interact.  In the island populations of finches in the Galapagos Islands.  The ancestral finches that first reached the islands after their volcanic origin presumably were few in number and consequently had fewer variations in the total "gene pool" than the larger continental populations from which they originated.  When new genes or combinations arose, that were better suited to survival on the various islands with varied conditions, selection might act more rapidly than in enormous potentially interbreeding continental populations.  Such selection can also operate on small semi-isolated populations of continental populations on the periphery of the species range.

Lock and key genitalia

Insect species have an effective way of preventing interbreeding between previously interbreeding populations.  Because of the need for, or value of, internal fertilization for reproduction of terrestrial adults there has been the evolution of specialized male organs for introducing sperm into the reproductive system of the females.  Selection has made the male and female genital openings of the semi-rigid exoskeleton match up in a "lock and key" arrangement of various shapes dependent on species.

The lock and key relationship of male and female genitalia presumably becomes better established over time so closely related species can no longer have cross-fertilization possible.  Other mechanisms may have helped the reproductive isolation occur.  Geographic isolation is not the only spatial mechanism to facilitate reproductive isolation.

Micro-habitat isolation

Some butterfly species in tropical forests isolate themselves from other similar species living in the same region, but at different elevations from the ground, some near the ground, other high in the canopy, and still others at an intermediate height.  Other insects may isolate themselves by their preference for a single species of plant species.  Many animals have parasitic species of insects found only on their species.  Various environmental conditions are often required by small organisms, sometimes in very small patches within what is a generally similar habitat by casual inspection; moisture, nutrients, soil particle size and texture, and chemical factors are features of micro-habitats that determine suitable environment for small organisms.

Imprinting

Lorenz discovered imprinting when he found that young geese responded to the first large moving animal they see after hatching as the object to follow.  They would follow him instead of the mother goose if they saw him first.  Imprinting of various types may occur at other times in the life cycle

A similar phenomenon occurs with the imprinting of the olfactory cues of a stream being imprinted on young salmon as guides to return to the same stream to breed.  It is thought that some insects preferentially lay eggs on the same plant type they fed upon as larval insects; a few times of use of a different plant variety could lead to separate evolutionary lines of the same species.

Temporal isolation

When the breeding season is extended over time it is possible for new species to evolve from populations separated by time of breeding.  I think this may have been a factor in evolution of species of isopods in Tasmania when the life cycle took three years for production of a new brood.  Cross-breeding would be less likely and three new species could evolve, especially when adults did not survive for a second breeding season.  Some intertidal populations of invertebrates have pairs of closely related species with separate reproductive seasons.  Many marine species have external fertilization and would benefit from a short breeding season giving specialized predators less time to prey upon them.

Pheromones

Chemical signals species give off include sex attractants given off by the females to facilitate their being found by the opposite sex.  Some tortricid moth species have been found to have sex attractants composed of two or three chemical components; ratios of the different compounds were different in each species and males only responded to the ratio characteristic of the species.

Pheromones of insects include other behavioral controls.  Formic acid is an alarm pheromone common to most species of ants; in fact, their family name, Formicidae, is based on that fact.

Beetle speciation

Beetles have more species than any other order of animals.  Their especially thick exoskeleton made the lock and key genitalia more effective in preserving the genetic isolation of new species once other adaptions became specialized.  The appearance of the new species may be almost identical to related species, something less common among related vertebrate species.  The hardened first pair of wings of beetles adapts them for survival without damage to their underlying membranous wings when crawling in forest floor debris.

Genetic factors in speciation

The important role of the genes in controlling development and function of organisms may be complicated by the complex life cycles of those with complete metamorphosis from larvae to pupae to adult.  It would seem that a lesser sequence of shifting controls would be found in insects with a gradual metamorphosis from wingless stages otherwise similar but preceding the adult stage.  The hormones regulating such changes have some parallels with vertebrate hormones.

Population size may affect the rate of evolutionary change although the loss of a better gene can occur by chance from mortality unrelated to a gene's value.  Local and/or broad scale catastrophes can ignore the fitness of a genome.  Barring loss of all with a better gene, it will probably become the most prevalent gene in a small population sooner than in a large population; if it is not lost, it will eventually be the norm in both.

The advantage of a complex life cycle

Most insects have a sequence of stages from egg, to larva, to pupa to adult.  The complexity might seem like a disadvantage exposing them to many different hazards during the course of their life cycle.  But consider the different ways they have developed to survive winter in temperate regions.  A species may overwinter in the egg stage, hidden away from predators and not a target food item for birds or other predators specializing on eating the adult or larval stage.

The sequence of stages through the year are less likely to enable excessive buildup of a predator population specialized to feed on one or two of the the stages.  When all stages are present at the same time, their different requirements may isolate them from competition with the other stages in feeding; it may also enable species survival by providing replacements if a particular stage has excessive predation.

An advantage of proper timing of stages of the insect species occurs when growth of the individual and its food organism, whether plant or animal, is at an optimal stage of growth.  Many insects feed on dead and/or decaying organisms that are present in accumulations soil or aquatic sediments.  Termites utilize wood effectively because they have symbiotic protozoans and bacteria in their gut that enable them to utilize cellulose, a plant material not digestible by large animals without such symbionts.

Speciation

Speciation is undoubtedly continuing among insect groups.  Most orders and families are quite ancient in their origin.  Fossils of insects much like those today have been found that are as ancient as the dinosaurs.

The previous post, http://evolutioninsights.blogspot.com/2015/10/evolution-insect-size.html , gives a probable reason insects have remained small.  Their small size has been a factor for their successful speciation into the largest number of species of any group of comparable sized organisms, with numbers of individuals far exceeding those of vertebrates and advanced invertebrates.

It is amazing that such diversity can be packed into the same adult body format- a head with compound eyes, antennae, and mouthparts; a thorax of three segments, three pair of legs and often two pair of wings; and an abdomen of about ten segments usually lacking appendages but bearing the genitalia.

There is great diversity in the specialization that insects have evolved for survival. It is a topic that could provide information for many interesting posts.

Joseph G. Engemann     Kalamazoo, Michigan     October 12, 2015

EVOLUTION: INSECT SIZE

INSECT EVOLUTION, SIZE LIMITATION CAUSES

There may be several reasons insects are all rather small.  But the principle reason limiting size of insects is the design of their respiratory system.  A clue to this may be found in comparison of insects and crustaceans.
Crustaceans also have an exoskeleton that must be shed for growth to occur, but they can be many times larger than any insect.  There are no truly marine species of insects beyond shoreline regions.  But large crustaceans are found in the ocean, in freshwater, and upon land.  So molting and the support provided by living in water are not the answer.

Both have blood in the body cavity. Blood enters the heart and is pumped out through arteries, bathing the organs in the body cavity as it returns to be pumped out again.  In the crustaceans the blood entering gills or flattened appendages is oxygenated as carbon dioxide is discharged.  That respiratory exchange takes place in the interior of the insect as the blood passes around the smaller tubules of the tracheal system.

The tracheal system is a system of tubules with openings on the surface of the insect.  The interior surface of the tracheal tubes and tubules is continuous with the outer chitinous exoskeleton layer covering the insect.  The tubules are closed on the end inside the insect.  The finer terminations may be water filled when the insect is inactive.  Increased numbers of solute molecules due to metabolic activity can cause water to leave the tubules and assist movement of oxygen into the blood.  The highly branched system's fine terminations in all parts of the body substitute for the need for a capillary network of a circulatory system.

The main factor for size limitation

Two facts may be responsible for limiting the insect size due to tracheal system function.  1- large size means greater hydrostatic pressure on the tubules may collapse them and inhibit respiration.  2 - large size means greater length of tubules may slow diffusion of oxygen into the regions more distant from the spiracle on the exterior surface.

The limitations of the circulatory and respiratory system and the resultant small size may also be a factor in preventing warm-blooded insects from developing.  Some larger insect such as certain bees and moths have larger bodies and ability to sustain a warmer temperature than the environment by "furry" bodies and a high rate of activity.

It is of interest that even aquatic insects have air-filled tracheal systems.  Several different mechanisms are used in different aquatic insects to oxygenate those tracheal systems.

It is likely that a crustacean ancestor, perhaps now extinct, gave rise to insects.

Small organisms have the advantages- of finding more hiding places, needing less food per individual, reaching adult size more rapidly, higher reproductive potential, and as is evident around us - having more species and specializations exist in a small area.

Joseph G. Engemann     Kalamazoo, Michigan   October 8, 2015